Method and device for cmos image sensing with multiple gate oxide thicknesses

ABSTRACT

A method and device for image sensing. The method includes forming a first well and a second well in a substrate, forming a gate oxide layer with at least a first part and a second part on the substrate, and depositing a first gate region and a second gate region on the gate oxide layer. The first part of the gate oxide layer is associated with a first thickness, and the second part of the gate oxide layer is associated with a second thickness. The first thickness and the second thickness are different. The first gate region is located on the first part of the gate oxide layer associated with the first thickness, while the second gate region is located on both the first part of the gate oxide layer associated with the first thickness and the second part of the gate oxide layer associated with the second thickness. The first gate region is associated with the first well, and the second gate region is associated with the second well. Additionally, the method includes forming a third well in the substrate, implanting a first plurality of ions to form a first lightly doped source region and a first lightly doped drain region in the first well, implanting a second plurality of ions to form at least a second lightly doped drain region in the second well, and implanting a third plurality of ions to form a source in the second well.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No. 200910247497.6, filed Dec. 29, 2009, which is commonly owned and incorporated herein by reference for all purposes.

Additionally, this application is related to U.S. patent application Ser. No. 11/185,444, filed Jul. 19, 2005, now U.S. Pat. No. 7,335,546, which is commonly assigned and incorporated by reference herein for all purposes.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to integrated circuits and the processing for the manufacture of semiconductor devices. More particularly, embodiments of the invention provide a method and device with multiple gate oxide thicknesses. Merely by way of example, the invention has been applied to CMOS image sensing. But it would be recognized that the invention has a much broader range of applicability.

Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. One such type of IC is a CMOS imaging system. The CMOS imaging system can be fabricated on standard silicon production lines and is therefore inexpensive to make. Additionally, the CMOS image sensor consumes low power and is especially suitable for portable applications.

Specifically, a CMOS image sensor converts a light signal into an electrical signal, whose intensity is related to the light intensity. FIG. 1 is a simplified diagram of a conventional CMOS image sensor. The CMOS image sensor 100 corresponds to one pixel and includes a reset transistor 110, a photodiode 120, a source follower 130, a selecting transistor 140, and a bias resistor 150. The photodiode 120 receives a light signal and generates a photocurrent from a node 160 to a node 170. Additionally, a leakage current also flows through the photodiode 120 in the same direction. One source of the leakage current is the source region of the reset transistor 110, which is connected to the photodiode 120.

FIG. 2 is a simplified conventional diagram of the reset transistor 110 and the photodiode 120. The photodiode 120 includes an active region 210, and the reset transistor 110 includes a source region 220, a drain region 230, and a gate region 240. The source region 220 forms a junction with the substrate or a well in the substrate, and the junction usually experiences certain leakage. The leakage is usually passed to the active region 210 and contributes to the leakage current of the photodiode 120. A large leakage current adversely affects the performance of the CMOS image sensor.

FIG. 3( a) is another simplified conventional diagram for the reset transistor 110 and the photodiode 120. The photodiode 120 includes a diode diffusion region 310 formed under a gate oxide region 320. The reset transistor 110 includes a source region 330, a drain region 340, and a gate region 350. Since dark current is a main issue associated with a CMOS image sensor, low leakage current of the CMOS image sensor is needed. The current leakage from the reset transistor 110 source region 330 to the gate region 350 due to similar phenomenon as gate induced drain leakage (GIDL) effect is one of the main sources for this leakage current. Reduction of the electric field in this area is needed to reduce GIDL effect.

FIG. 3( b) is yet another simplified convention diagram for the reset transistor 110 and the photodiode 120. The photodiode 120 includes a diode diffusion region 355 formed under a field oxide region 360. The reset transistor 110 includes a source region 370, a drain region 380, and a gate region 390. The source region 370 is connected to the diode diffusion region 355 and formed with a deep source implantation penetrating the gate region 390. The gate region 390 is not aligned with the source region, so the reliability of the CMOS image sensor usually deteriorates. Any misalignment would affect the Cgs (CMOS gate-to-source capacitance) uniformity, and in turn the output signal reset level uniformity, throughout the wafer, since Cgs has impact on the reset output signal level.

From the above, it is seen that an improved technique for CMOS image sensor is desired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a method and device with improved dark leakage current. Merely by way of example, the invention has been applied to CMOS image sensing. But it would be recognized that the invention has a much broader range of applicability.

In a specific embodiment, the invention provides a method for making an image sensor. The method includes forming a first well and a second well in a substrate, forming a gate oxide layer with at least a first part and a second part on the substrate, and depositing a first gate region and a second gate region on the gate oxide layer. The first part of the gate oxide layer is associated with a first thickness, and the second part of the gate oxide layer is associated with a second thickness. The first thickness and the second thickness are different. The first gate region is located on the first part of the gate oxide layer associated with the first thickness, while the second gate region is located on both the first part of the gate oxide layer associated with the first thickness and the second part of the gate oxide layer associated with the second thickness. The first gate region is associated with the first well, and the second gate region is associated with the second well. Additionally; the method includes forming a third well in the substrate. The first well and the second well are associated with a CMOS, and the third well is associated with a photodiode.

According to another embodiment of the present invention, a device for image sensing includes a semiconductor substrate. Additionally, the device includes a first well, a second well, and a third well in the semiconductor substrate. The first well and the second well are associated with a CMOS, and the third well is associated with a photodiode. Moreover, the device includes a gate oxide layer with at least a first part and a second part on the semiconductor substrate including the third well. The first gate oxide layer part is associated with a first thickness, and the second gate oxide layer part is associated with a second thickness. The first thickness and the second thickness are different. Also, the device includes a first gate region and a second gate region on the gate oxide. The first gate region and the second gate region are associated with the first well and the second well respectively.

Many benefits are achieved by way of the present invention over conventional techniques. For example, the present technique provides an easy to use process that relies upon conventional technology. Some embodiments of the present invention provide a thicker gate oxide film, which helps reduce the electric field between reset transistor gate and source/drain diffusion (LDD) that is connected to the photodiode diffusion region. This reduction in electric field decreases the leakage between the gate and source/drain which is connected to the photodiode. As a result of the increased gate oxide thickness, the Cgs (CMOS gate-to-source capacitance) is reduced as well, which can benefit output signal noise reduction. Certain embodiments of the present invention rely on self-aligned implantation, which would maintain the CMOS gate-to-source capacitance uniformity throughout the wafer. It will not be affected by misalignment. Some embodiments of the present invention provide a source that is substantially self-aligned with a spacer and connected to a photodiode. Certain embodiments of the present invention improve uniformity of CMOS gate-to-source capacitance and reduce the gate-to-source capacitance and the source sheet resistance. Additionally, the method provides a process that is compatible with conventional process technology without substantial modifications to conventional equipment and processes. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more throughout the present specification and more particularly below.

Various additional objects, features and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a conventional CMOS image sensor;

FIG. 2 is a simplified conventional diagram of a reset transistor and a photodiode;

FIG. 3( a) is another simplified conventional diagram of a reset transistor and a photodiode;

FIG. 3( b) is yet another simplified conventional diagram of a reset transistor and a photodiode;

FIG. 4 is a simplified method for forming an image sensor according to an embodiment of the present invention;

FIG. 5 shows a process for well formation according to an embodiment of the present invention;

FIG. 6 shows a process for gate oxide formation according to an embodiment of the present invention;

FIG. 7 shows a process for polysilicon deposition according to an embodiment of the present invention;

FIG. 8 shows a process for polysilicon etching according to an embodiment of the present invention;

FIG. 9 shows a process for photodiode well formation according to an embodiment of the present invention;

FIG. 9( a) is a simplified diagram of a process for photodiode well formation according to an alternative embodiment of the present invention;

FIG. 10 shows a process for forming lightly doped regions and spacers according to an embodiment of the present invention;

FIG. 10( a) shows a process for forming lightly doped regions according to another embodiment of the present invention;

FIG. 11 shows a process for source region formation according to an embodiment of the present invention;

FIG. 11( a) shows a process for reset transistor source region formation according to another embodiment of the present invention

FIG. 11( b) shows a process for spacer formation according to another embodiment of the present invention

FIG. 12 shows a process for forming heavily doped regions according to an embodiment of the present invention;

FIG. 12( a) shows a process for forming heavily doped regions according to another embodiment of the present invention;

FIG. 13 is a simplified device for image sensing according to an embodiment of the present invention;

FIG. 13( a) is a simplified device for image sensing according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate to integrated circuits and the processing for the manufacture of semiconductor devices. More particularly, embodiments of the invention provide a method and device with multiple gate oxide thicknesses. Merely by way of example, the invention has been applied to CMOS image sensing. But it would be recognized that the invention has a much broader range of applicability.

FIG. 4 is a simplified method for forming an image sensor according to an embodiment of the present invention. The method 400 includes the following processes:

-   -   1. Process 405 for forming transistor wells;     -   2. Process 410 for forming gate oxide layer with two different         thicknesses;     -   3. Process 420 for depositing polysilicon;     -   4. Process 430 for etching polysilicon;     -   5. Process 440 for forming photodiode well;     -   6. Process 450 for forming lightly doped regions and spacers;     -   7. Process 460 for forming transistor source region;     -   8. Process 470 for forming heavily doped regions.

The above sequence of processes provides a method according to an embodiment of the present invention. Other alternatives can also be provided where processes are added, one or more processes are removed, or one or more processes are provided in a different sequence without departing from the scope of the claims herein. For example, additional processes are provided to form a source follower, a selecting transistor, and a bias resistor. Future details of the present invention can be found throughout the present specification and more particularly below.

At the process 405, transistor wells are formed. FIG. 5 shows process 405 for well formation according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

As shown in FIG. 5, an n-type well 510 and a p-type well 520 are formed in a semiconductor substrate 530. In one embodiment, the wells 510 and 520 are at least in part confined by shallow trench isolations 540. In another embodiment, the n-type well 510 and the p-type well 520 are formed with ion implantation processes and/or diffusion processes. The n-type dopants may be arsenic and/or phosphorous, and the p-type dopants may be boron. In yet another embodiment, the depth of the n-type well 510 ranges from 0.5 μm to 1.0 μm, and the doping concentration of the n-type well 510 ranges from 5×10¹⁶ cm⁻³ to 3×10¹⁷ cm⁻³. The depth of the p-type well 520 ranges from 0.5 μm to 1.0 μm, and the doping concentration of the p-type well 510 ranges from 5×10¹⁶ cm⁻³ to 3×10¹⁷ cm⁻³. In yet another embodiment, the semiconductor substrate 530 is a silicon substrate.

At the process 410, a gate oxide layer with two different thicknesses is formed. FIG. 6 shows process 410 where a gate oxide layer with two different thicknesses, thickness one (gate oxide layer 550) and thickness two (gate oxide layer 560), is formed on the semiconductor substrate 530 according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, gate oxide layer 550 is thinner than gate oxide layer 560. In one embodiment, the gate oxide layers 550 and 560 include silicon oxide. The gate oxide layers 550 and 560 are grown or deposited. In another embodiment, the two-thickness gate oxide is grown using the conventional dual gate or triple gate process. In the conventional dual gate or triple gate process, first form a thin layer of gate oxide; then photo/etch to selectively remove oxide in the area where the gate oxide needs to be thin; finally grow more gate oxide. In yet another embodiment, the thickness of the thinner oxide layer 550 ranges from 15 Å to 75 Å, while the thickness of the thicker oxide layer 560 ranges from 50 Å to 150 Å. In still another embodiment, the thickness of the thicker oxide layer 560 is 20 Å to 30 Å greater than the thinner oxide layer 550.

At the process 420, a polysilicon layer 610 is deposited. FIG. 7 shows process 420 for polysilicon deposition according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 7, a polysilicon layer 610 is deposited on the gate oxide layer (550 and 560). In one embodiment, the deposition includes chemical vapor deposition, low pressure chemical vapor deposition, plasma enhanced chemical vapor deposition, and/or sputtering deposition. In another embodiment, the thickness of the polysilicon layer 610 ranges from 1800 Å to 2200 Å. In yet another embodiment, the polysilicon layer 610 is doped with either an n-type or a p-type dopant. The dopant concentration may range from 1×10¹⁸ cm⁻³ to 4×10¹⁹ cm⁻³.

At the process 430, the polysilicon layer 610 is etched. FIG. 8 shows process 430 for polysilicon etching according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 7, the polysilicon layer 610 is selectively etched to form the polysilicon gates 710 and 720, such that the polysilicon gate 710 is located on the gate oxide layer 550 while the polysilicon gate 720 is located on both the gate oxide layer 550 and the gate oxide layer 560. The etching process may include a dry etch and/or a wet etch. In one embodiment, the polysilicon gate 710 is located on the n-type well 510, and the polysilicon gate 720 is located on the p-type well 520.

At the process 440, a photodiode well is formed. FIG. 9 shows process 440 for photodiode well formation according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 9, a photodiode well 810 is formed in the semiconductor substrate 530. In one embodiment, the photodiode well 810 has p-type conductivity. In another embodiment, the photodiode well 810 is formed with an ion implantation process through the gate oxide layer 560. For example, the implant energy ranges from 100 KeV to 250 KeV, and the dose ranges from 10¹² to 10¹⁴ cm⁻². As another example, the ion implantation process is performed with a barrier layer. The barrier layer may be patterned by a photolithography process using a photo mask 820. As shown in FIG. 9, the barrier layer can substantially block any implanted ion from entering the n-type and p-type wells 510 and 520. For example, the barrier layer includes photoresist.

At the process 450, lightly doped regions and spacers are formed. FIG. 10 shows process 450 for forming lightly doped regions and spacers according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 10, lightly doped regions 910 and 912 are formed on both sides of the gate region 710, and lightly doped regions 920 and 922 are formed on both sides of the gate region 720. In one embodiment, the regions 910 and 912 are p-type, and the regions 920 and 922 are n-type. The region 920 is separated from the region 912 by a shallow trench isolation 540. In another embodiment, the depth for the lightly doped region 910 or 912 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³. The depth for the lightly doped regions 920 or 922 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³. In yet another embodiment, the lightly doped regions 910 and 912 each are used as a lightly doped source or drain region. The lightly doped regions 920 and 922 are used as a lightly doped drain region and a lightly doped source region respectively. In yet another embodiment, the lightly doped regions 910, 912, 920 and 922 each are formed by ion implantation and/or diffusion. Also as shown in FIG. 10, spacers 930 and 932 are formed for the gate region 710, and spacers 940 and 942 are formed for the gate region 720. In one embodiment, the spacers 930 and 932 are on at least part of the lightly doped regions 910 and 912 respectively. The spacers 940 and 942 are on at least part of the lightly doped regions 920 and 922 respectively.

At the process 460, a source region is formed. FIG. 11 shows process 460 for source region formation according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 11, a source region 1010 is formed in the semiconductor substrate 530. In one embodiment, the source region 1010 is formed with an ion implantation process through the gate oxide layer 560. For example, the implant energy ranges from 40 KeV to 80 KeV, and the dose ranges from 10³ to 10⁵ cm⁻². As another example, the ion implantation process is performed with a barrier layer. The barrier layer may be patterned by a photolithography process using a photo mask 1020. As shown in FIG. 11, the barrier layer, the gate region 720 and the spacer 942 can substantially block any implanted ion from entering the lightly doped regions 910, 912 and 920 and part of the lightly doped region 922. For example, the barrier layer includes photoresist.

At the process 470, heavily doped regions are formed. FIG. 12 shows process 470 for forming heavily doped regions according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 12, heavily doped regions 1110 and 1112 are formed on both sides of the gate region 710, and a heavily doped region 1120 is formed on only one side of the gate region 720. In one embodiment, the regions 1110 and 1112 are p-type, and the region 1120 is n-type. The region 1120 is separated from the region 1112 by a shallow trench isolation 540. In another embodiment, the depth for the heavily doped region 1110 or 1112 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. The depth for the heavily doped region 1120 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. In yet another embodiment, the heavily doped regions 1110 and 1112 each are used as a heavily doped source or drain region. The heavily doped region 1120 is used as a heavily doped drain region. In yet another embodiment, the heavily doped regions 1110, 1112 and 1120 each are formed by ion implantation and/or diffusion.

As discussed above and further emphasized here, FIG. 4 is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. FIG. 9( a) shows an alternative process for photodiode well formation according to another embodiment of the present invention. A photodiode well 810 is formed in the semiconductor substrate 530. For example, gate 720 overlaps a portion of the photodiode well 810. In one embodiment, the photodiode well 810 has n-type conductivity. In another embodiment, the photodiode well 810 is formed with an ion implantation process through the gate oxide layer 560. For example, the implant energy ranges from 100 KeV to 250 KeV, and the dose ranges from 10¹² to 10¹⁴ cm⁻². As another example, the ion implantation process is performed with a barrier layer. The barrier layer may be patterned by a photolithography process using a photo mask (not shown). As shown in FIG. 9( a), the barrier layer can substantially block any implanted ion from entering the n-type well 510 (not shown) and p-type well 520. In an embodiment, the barrier layer may allow ion implantation through part of polysilicon gate 720 to form an overlap between the photodiode well and the polysilicon gate. In an alternative embodiment, the overlap may be formed by lateral diffusion of the impurities in the photodiode well. Photodiode well 810 is associated with a source region of a transistor which includes polysilicon gate 720 and well 520.

As discussed above with reference to FIG. 9( a), polysilicon gate 720 extends over the photodiode well region 810 according to a specific embodiment of the present invention. The photodiode well 810 can be used as a source region. For example, the source region ion implantation as shown in FIG. 11 may be omitted. Of course, one of ordinary skill in the art would recognize many variations, alternatives, and modifications.

In yet another embodiment, the method 400 includes the following processes:

-   -   1. Process 405 for forming transistor wells;     -   2. Process 410 for forming gate oxide layer with two different         thicknesses;     -   3. Process 420 for depositing polysilicon;     -   4. Process 430 for etching polysilicon;     -   5. Process 440 for forming photodiode well;     -   6. Process 455 for forming lightly doped regions;     -   7. Process 463 for forming transistor source region;     -   8. Process 465 for forming spacers;     -   9. Process 475 for forming heavily doped regions.

Processes 405-440 are similar to the processes described above in connection with FIG. 4. At the process 455, lightly doped regions are formed. FIG. 10( a) shows process 455 for forming lightly doped regions according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 10( a), lightly doped regions 910 and 912 are formed on both sides of the gate region 710, and a lightly doped region 920 is formed on only one side of the gate region 720. In one embodiment, the regions 910 and 912 are p-type, and the region 920 is n-type. The region 920 is separated from the region 912 by a shallow trench isolation 540. In another embodiment, the depth for the lightly doped region 910 or 912 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³. The depth for the lightly doped region 920 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³. In yet another embodiment, the lightly doped regions 910 and 912 each are used as a lightly doped source or drain region. The lightly doped region 920 is used as a lightly doped drain region. In yet another embodiment, the lightly doped regions 910, 912 and 920 each are formed by ion implantation and/or diffusion.

At the process 463, a source region is formed. FIG. 11( a) shows process 463 for source region formation according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 11( a), a source region 1015 is formed in the semiconductor substrate 530. In one embodiment, the source region 1015 is formed with an ion implantation process through the gate oxide layer 560. For example, the implant energy ranges from 40 KeV to 80 KeV, and the dose ranges from 10³ to 10⁵ cm⁻². As another example, the ion implantation process is performed with a barrier layer. The barrier layer may be patterned by a photolithography process using a photo mask 1020. As shown in FIG. 11( a), the barrier layer and the gate region 720 can substantially block any implanted ion from entering the lightly doped regions 910, 912 and 920. For example, the barrier layer includes photoresist.

At the process 465, spacers are formed. FIG. 11( b) shows process 465 for spacer formation according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 11( b), spacers 930 and 932 are formed for the gate region 710, and spacers 940 and 942 are formed for the gate region 720. In one embodiment, the spacers 930 and 932 are on at least part of the lightly doped regions 910 and 912 respectively. The spacers 940 and 942 are on at least part of the lightly doped region 920 and the source region 1015 respectively.

At the process 475, heavily doped regions are formed. FIG. 12( a) shows process 475 for forming heavily doped regions according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 12( a), heavily doped regions 1110 and 1112 are formed on both sides of the gate region 710, and a heavily doped region 1120 is formed on only one side of the gate region 720. In one embodiment, the regions 1110 and 1112 are p-type, and the region 1120 is n-type. The region 1120 is separated from the region 1112 by a shallow trench isolation 540. In another embodiment, the depth for the heavily doped region 1110 or 1112 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. The depth for the heavily doped region 1120 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. In yet another embodiment, the heavily doped regions 1110 and 1112 each are used as a heavily doped source or drain region. The heavily doped region 1120 is used as a heavily doped drain region. In yet another embodiment, the heavily doped regions 1110, 1112 and 1120 each are formed by ion implantation and/or diffusion.

FIG. 13 is a simplified device for image sensing according to an embodiment of the present invention. The device 1200 includes the following components:

-   -   1. Substrate 1210;     -   2. Transistor wells 1220 and 1222;     -   3. Heavily doped regions 1230, 1232 and 1234;     -   4. Lightly doped regions 1236, 1237, 1238 and 1239;     -   5. Source region 1240;     -   6. Shallow trench isolations 1250 and 1252;     -   7. Gate oxide layer with two different thicknesses, thickness         one (gate oxide layer 1260) and thickness two (gate oxide layer         1265);     -   8. Gate regions 1270 and 1272;     -   9. Spacers 1280, 1282, 1284, and 1286;     -   10. Photodiode well 1290.

The above group of components provide a device according to an embodiment of the present invention. Other alternatives can also be provided where components are added, one or more components are removed, or one or more components are provided in a different arrangement without departing from the scope of the claims herein. For example, a source follower, a selecting transistor, and a bias resistor may also be provided to the device 1200. As another example, the device 1200 is fabricated according to the method 400 including at least the processes 450, 460 and 470. Further details of the present invention can be found throughout the present specification and more particularly below.

In one embodiment, the substrate 1210 is a semiconductor substrate, such as a silicon substrate. The transistor wells 1220 and 1222 are n-type and p-type respectively. For example, the depth of the n-type well 1220 ranges from 0.5 μm to 1.0 μm, and the doping concentration of the n-type well 1220 ranges from 5×10¹⁶ cm⁻³ to 3×10¹⁷ cm⁻³. The depth of the p-type well 1222 ranges from 0.5 μm to 1.0 μm, and the doping concentration of the p-type well 1222 ranges from 5×10¹⁶ cm⁻³ to 3×10¹⁷ cm⁻³.

The heavily doped regions 1230 and 1232 are formed on both sides of the gate region 1270 and are substantially self-aligned with the spacers 1280 and 1282 respectively. The heavily doped region 1234 is formed on only one side of the gate region 1272 and is substantially self-aligned with the spacer 1284. In one embodiment, the regions 1230 and 1232 are p-type, and the region 1234 is n-type. The region 1232 is separated from the region 1234 by the shallow trench isolation 1252. In another embodiment, the depth for the heavily doped region 1230 or 1232 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. The depth for the heavily doped region 1234 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³.

The lightly doped regions 1236 and 1238 are located on both sides of the gate region 1270 and are substantially self-aligned with the gate region 1270. The lightly doped regions 1239 and 1237 are located on both sides of the gate region 1272 and are substantially aligned with the gate region 1272. In one embodiment, the regions 1236 and 1238 are p-type, and the regions 1239 and 1237 are n-type. The region 1238 is separated from the region 1239 by the shallow trench isolation 1252. In another embodiment, the depth for the lightly doped region 1236 or 1238 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³. The depth for the lightly doped region 1239 or 1237 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³.

The source region 1240 is located in the semiconductor substrate 1210. In one embodiment, the source region 1240 is n-type. The depth for the source region 1240 ranges from 2000 Å to 3500 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 1×10¹⁹ cm⁻³. In another embodiment, the depth of the source region 1240 is different from the depth of the lightly doped source region 1237 and/or the depth of the heavily doped source region 1234. In yet another embodiment, the depth of the source region 1240 is larger than the depth of the lightly doped source region 1237. In yet another embodiment, the source region 1240 is substantially aligned with the spacer 1286.

A gate oxide layer with two different thicknesses, thickness one (gate oxide layer 1260) and thickness two (gate oxide layer 1265), is formed on the semiconductor substrate 1210. For example, gate oxide layer 1260 is thinner than gate oxide layer 1265. In one embodiment, the gate oxide layers 1260 and 1265 include silicon oxide. The gate oxide layers 1260 and 1265 are grown or deposited. In another embodiment, the two-thickness gate oxide is grown using the conventional dual gate or triple gate process. In the conventional dual gate or triple gate process, first form a thin layer of gate oxide; then photo/etch to selectively remove oxide in the area where the gate oxide needs to be thin; finally grow more gate oxide. In yet another embodiment, the thickness of the thinner oxide layer 1260 ranges from 15 Å to 70 Å, while the thickness of the thicker oxide layer 1265 ranges from 50 Å to 150 Å. In still another embodiment, the thickness of the thinner oxide layer 1260 ranges from 150 Å to 400 Å. In one embodiment, the gate region 1270 is located on the gate oxide layer 1260, while the gate region 1272 is located on both the gate oxide layer 1260 and the gate oxide layer 1265. In another embodiment, the gate region 1270 is over the n-type well 1220, and the gate region 1272 is over the p-type well 1222. In yet another embodiment, the gate regions 1270 and 1272 each are doped either n-type or p-type. The dopant concentration may range from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. In still another embodiment, the thickness of the gate region 1270 or 1272 ranges from 1800 Å to 2200 Å.

The spacers 1280 and 1282 are next to the gate regions 1270, and the spacers 1284 and 1286 are next to the gate region 1272. In one embodiment, the spacers 1280 and 1282 are on at least part of the lightly doped regions 1236 and 1238 respectively. The spacers 1284 and 1286 are on at least part of the lightly doped regions 1239 and 1237 respectively. The thickness 1288 for the spacer 1280, 1282, 1284 or 1286 ranges from 1200 Å to 1800 Å. In another embodiment, the source region 1240 is substantially aligned with the spacer 1286.

The photodiode well 1290 is located under the gate oxide layer 1265 and in the semiconductor substrate 1210. In one embodiment, the photodiode well 1290 has p-type conductivity. In yet another embodiment, the photodiode well has a thickness ranging from 3000 Å to 5000 Å.

FIG. 13( a) is a simplified device for image sensing according to another embodiment of the present invention. The device 1300 includes the following components:

-   -   1. Substrate 1210;     -   2. Transistor wells 1220 and 1222;     -   3. Heavily doped regions 1230, 1232 and 1234;     -   4. Lightly doped regions 1236, 1238 and 1239;     -   5. Source region 1340;     -   6. Shallow trench isolations 1250 and 1252;     -   7. Gate oxide layer with two different thicknesses, thickness         one (gate oxide layer 1260) and thickness two (gate oxide layer         1265);     -   8. Gate regions 1270 and 1272;     -   9. Spacers 1280, 1282, 1284, and 1286;     -   10. Photodiode well 1290.

The above group of components provide a device according to an embodiment of the present invention. Other alternatives can also be provided where components are added, one or more components are removed, or one or more components are provided in a different arrangement without departing from the scope of the claims herein. For example, a source follower, a selecting transistor, and a bias resistor may also be provided to the device 1300. As another example, the device 1300 is fabricated according to the method 400 including at least the processes 455, 463, 465 and 475. Future details of the present invention can be found throughout the present specification and more particularly below.

In one embodiment, the substrate 1210 is a semiconductor substrate, such as a silicon substrate. The transistor wells 1220 and 1222 are n-type and p-type respectively. For example, the depth of the n-type well 1220 ranges from 0.5 μm to 1.0 μm, and the doping concentration of the n-type well 1220 ranges from 5×10¹⁶ cm⁻³ to 3×10¹⁷ cm⁻³. The depth of the p-type well 1220 ranges from 0.5 μm to 1.0 μm, and the doping concentration of the p-type well 1222 ranges from 5×10¹⁶ cm⁻³ to 3×10¹⁷ cm⁻³.

The heavily doped regions 1230 and 1232 are formed on both sides of the gate region 1270 and are substantially self-aligned with the spacers 1280 and 1282 respectively. The heavily doped region 1234 is formed on only one side of the gate region 1272 and is substantially self-aligned with the spacer 1284. In one embodiment, the regions 1230 and 1232 are p-type, and the region 1234 is n-type. The region 1232 is separated from the region 1234 by the shallow trench isolation 1252. In another embodiment, the depth for the heavily doped region 1230 or 1232 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. The depth for the heavily doped region 1234 ranges from 500 Å to 2000 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³.

The lightly doped regions 1236 and 1238 are located on both sides of the gate region 1270 and are substantially self-aligned with the gate region 1270. The lightly doped region 1239 is located on only one side of the gate region 1272 and is substantially aligned with the gate region 1272. In one embodiment, the regions 1236 and 1238 are p-type, and the region 1239 is n-type. The region 1238 is separated from the region 1239 by the shallow trench isolation 1252. In another embodiment, the depth for the lightly doped region 1236 or 1238 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³. The depth for the lightly doped region 1239 ranges from 500 Å to 1500 Å, and the dopant concentration ranges from 1×10¹⁷ cm⁻³ to 3×10¹⁸ cm⁻³.

The source region 1340 is located in the semiconductor substrate 1210. In one embodiment, the source region 1340 is n-type. The depth for the source region 1340 ranges from 2000 Å to 3500 Å, and the dopant concentration ranges from 1×10¹⁸ cm⁻³ to 1×10¹⁹ cm⁻³. In another embodiment, the depth of the source region 1340 is different from the depth of the lightly doped source region 1239 and/or the depth of the heavily doped source region 1234. In yet another embodiment, the depth of the source region 1340 is larger than the depth of the lightly doped source region 1239. In yet another embodiment, the source region 1340 is substantially aligned with the gate region 1272.

A gate oxide layer with two different thicknesses, thickness one (gate oxide layer 1260) and thickness two (gate oxide layer 1265), is formed on the semiconductor substrate 1210. For example, gate oxide layer 1260 is thinner than gate oxide layer 1265. In one embodiment, the gate oxide layers 1260 and 1265 include silicon oxide. The gate oxide layers 1260 and 1265 are grown or deposited. In another embodiment, the two-thickness gate oxide is grown using the conventional dual gate or triple gate process. In the conventional dual gate or triple gate process, first form a thin layer of gate oxide; then photo/etch to selectively remove oxide in the area where the gate oxide needs to be thin; finally grow more gate oxide. In yet another embodiment, the thickness of the thinner oxide layer 1260 ranges from 15 Å to 70 Å, while the thickness of the thicker oxide layer 1265 ranges from 50 Å to 150 Å. In still another embodiment, the thickness of the thinner oxide layer 1260 ranges from 150 Å to 400 Å. In one embodiment, the gate region 1270 is located on the gate oxide layer 1260, while the gate region 1272 is located on both the gate oxide layer 1260 and the gate oxide layer 1265. In another embodiment, the gate region 1270 is over the n-type well 1220, and the gate region 1272 is over the p-type well 1222. In yet another embodiment, the gate regions 1270 and 1272 each are doped either n-type or p-type. The dopant concentration may range from 1×10¹⁸ cm⁻³ to 2×10¹⁹ cm⁻³. In still another embodiment, the thickness of the gate region 1270 or 1272 ranges from 1800 Å to 2200 Å.

The spacers 1280 and 1282 are next to the gate regions 1270, and the spacers 1284 and 1286 are next to the gate region 1272. In one embodiment, the spacers 1280 and 1282 are on at least part of the lightly doped regions 1236 and 1238 respectively. The spacers 1284 and 1286 are on at least part of the lightly doped region 1239 and the source region 1340 respectively. The thickness 1288 for the spacer 1280, 1282, 1284 or 1286 ranges from 1200 Å to 1800 Å.

The photodiode well 1290 is located under the gate oxide layer 1265 and in the semiconductor substrate 1210. In one embodiment, the photodiode well 1290 has p-type conductivity. In yet another embodiment, the photodiode well has a thickness ranging from 3000 Å to 5000 Å.

It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. 

1. A method for making an image sensor, the method comprising: forming a first well and a second well in a substrate; forming a gate oxide layer including at least a first part and a second part on the substrate, the first part being associated with a first thickness, the second part being associated with a second thickness; depositing a first gate region and a second gate region on the gate oxide layer, the first gate region being associated with the first well, the second gate region being associated with the second well; and forming a third well in the substrate; wherein: the first well and the second well are associated with a CMOS; the third well is associated with a photodiode; the first gate region is located on the first part associated with the first thickness; the second gate region is located on the first part associated with the first thickness and the second part associated with the second thickness; and the first thickness and the second thickness are different.
 2. The method of claim 1, wherein the first gate region is not located on the second part associated with the second thickness.
 3. The method of claim 2, wherein the first thickness is less than the second thickness.
 4. The method of claim 2, wherein the third well is located under the second part associated with the second thickness.
 5. The method of claim 1, wherein a source associated with the CMOS is substantially self-aligned with the second gate region.
 6. The method of claim 1, wherein the depositing the first gate region and the second gate region comprises: depositing a polysilicon layer on the gate oxide layer; and removing at least a part of the polysilicon layer to form the first gate region and the second gate region.
 7. The method of claim 1, wherein: the first well is associated with n-type conductivity; and the second well and the third well each are associated with p-type conductivity.
 8. The method of claim 1, wherein the third well is a source region of an MOS transistor associated with the second well region.
 9. The method of claim 8, wherein the third well is separated from the second well.
 10. The method of claim 8, wherein the third well overlaps with the second well.
 11. The method of claim 1, wherein the first well is characterized by a n-type conductivity, the second well is characterized by a p-type conductivity, and the third well is characterized by an n-type conductivity.
 12. A device for image sensing, the device comprising: a semiconductor substrate; a first well, a second well, and a third well in the semiconductor substrate, the first well and the second well being associated with a CMOS, the third well being associated with a photodiode; a gate oxide layer including at least a first part and a second part on the semiconductor substrate including the third well, the first part being associated with a first thickness, the second part being associated with a second thickness; a first gate region and a second gate region on the gate oxide, the first gate region and the second gate region being associated with the first well and the second well respectively; and wherein the first gate region is located on the first part associated with the first thickness; the second gate region is located on the first part associated with the first thickness and the second part associated with the second thickness; and the first thickness and the second thickness are different.
 13. The device of claim 12, wherein the first gate region is not located on the second part associated with the second thickness.
 14. The device of claim 13, wherein the first thickness is less than the second thickness.
 15. The device of claim 13, wherein the third well is located under the second part associated with the second thickness.
 16. The device of claim 12, wherein a source associated with the CMOS is substantially self-aligned with the second gate region.
 17. The device of claim 12, wherein: the first gate region includes a first polysilicon layer on the gate oxide layer; and the second gate region includes a second polysilicon layer on the gate oxide layer.
 18. The device of claim 12, wherein: the first well is associated with n-type conductivity; and the second well and the third well each are associated with p-type conductivity.
 19. The device of claim 12, wherein the third well is a source region of an MOS transistor associated with the second well region.
 20. The device of claim 19, wherein the third well is separated from the second well.
 21. The device of claim 19, wherein the third well overlaps with the second well.
 22. The device of claim 12, wherein the first well is characterized by a n-type conductivity, the second well is characterized by a p-type conductivity, and the third well is characterized by an n-type conductivity. 